yacht: Split into one deftest per test

[tasxatzial]

Also

• Updated the yacht.clj starter file with a proper function template
• Added the missing .toml file

[github-actions]

This PR touches files which potentially affect the outcome of the tests of an exercise. This will cause all students' solutions to affected exercises to be re-tested.

If this PR does not affect the result of the test (or, for example, adds an edge case that is not worth rerunning all tests for), please add the following to the merge-commit message which will stops student's tests from re-running. Please copy-paste to avoid typos.

[no important files changed]

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[ErikSchierboom]

Please merge with [no important files changed]